The present invention relates generally to methods and arrangements for calibrating illumination assemblies to obtain desired white-point, color balance and/or intensity. More specifically, the invention relates to using electronic storage devices and/or photodetectors and electronic circuitry to vary the current supplied to illumination devices such as light-emitting diodes, thus providing a calibrated light source for display applications.
In micro-display applications utilizing tri-color RGB (red, green and blue) light-emitting diode (LED) assemblies to illuminate a display panel, LED part-to-part illumination variation results in inconsistent brightness, white-point and color balance. Every LED""s illumination output as a function of current is different, and each LED""s illumination response to current across its entire current-controlled operating range may be non-linear. Manufacturing LEDs within tighter tolerances and more closely matching the three LED colors in a single assembly, thereby providing a more stable white-point and/or color balance, would be unnecessarily expensive, and would nevertheless provide unsatisfactory results.
Referring initially to FIG. 1, a prior art display system 100 providing a partial solution to the above-described problem will be described. Display system 100 includes a light modulating display 102, an illumination device 104, which provides the light source for display 102, and an adjustable current source 106 electrically connected to illumination device 104. Adjustable current source 106 is manually adjusted during manufacturing in order to cause illumination device 104 to provide calibrated light. The adjustment takes place by comparing the illumination output of illumination device 104 to a reference intensity and adjusting current source 106 until the illumination output of illumination device 104 matches the reference intensity. If illumination device 104 contains more than one light source, the process is repeated for each light source.
Display system 100 further includes a controller 108 and a display information input 110. During operation of display system 100, controller 108 receives display information via input 110 and determines the current to be supplied to illumination device 104. The setting made during manufacturing to adjustable current source 106 causes the current to vary proportionally to the setting, thereby providing partially calibrated light. Because the adjustment made to adjustable current source 106 during manufacturing calibrates the illumination output of illumination device 104 for only a single intensity, this system does not correct the non-linear illumination response to current of illumination device 104 across the device""s entire current-controlled operating range.
Display system 100 includes the additional limitation that adjustable current source 106 must be manually set during manufacturing. Having to manually calibrate the current source increases the cost of producing such a device. FIG. 2 illustrates a display system that overcomes this particular limitation.
Referring now to FIG. 2, a second prior art display system 120 will be described using like reference numbers for like components. Display system 120 includes a voltage source 122 and an adjustable resistor 124. Adjustable resistor 124 may be a laser trim resistor that is capable of being adjusted during manufacturing using an automated process to provide the desired intensity for a specific voltage. While this method overcomes one limitation of display system 100 by allowing the calibration to be accomplished by automated means during manufacturing, display system 120 similarly fails to correct the non-linear illumination response to current of illumination device 104 across its entire current-controlled operating range. Further, neither display system 100 nor display system 120 is capable of correcting illumination device variations that occur after manufacturing, such as illumination device aging.
The present invention discloses arrangements and methods for calibrating illumination devices to reduce both pre- and post-manufacturing variations, including non-linear illumination output as a function of current across the current-controlled operating range and illumination device aging.
As will be described in more detail hereinafter, a display system including an arrangement for calibrating an input-driven illumination device is disclosed. The display system includes a spatial light modulator divided into an array of individually controllable pixels and an input-driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto the modulator, depending on the input. The display system further includes an arrangement adapted for connection with the illumination device for providing to the illumination device a specific input for a desired intensity level of the light, the specific input being provided from calibration information particular to the illumination device. The arrangement further includes a memory device for storing the calibration information.
A method of operating a display system as described above includes determining calibration information for an input driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto a light modulator, depending on the input. The method further includes storing the calibration information in a memory device and establishing a specific input for a desired intensity level of the light from the calibration information. The method further includes providing the specific input to the illumination device, and directing the light of the desired intensity level onto the light modulator.
As will be described in more detail hereinafter, an illumination assembly, including calibration information is also disclosed. The illumination assembly includes an input-driven illumination device which is adapted to receive a variable input and which is configured to produce light of variable intensity depending on the input. The illumination assembly further includes an arrangement including a memory device for storing calibration information and generating from the information a specific input for causing the illumination device to produce light of a particular intensity. The arrangement is adapted to be connected with the illumination device such that the latter receives the specific input.
In another embodiment of a display system, the display system includes a light modulator and an input-driven illumination device which has been pre-calibrated to provide light of a given intensity in response to a particular input and which is configured to direct the light onto the modulator. The display system further includes an electronic storage arrangement for storing a value which corresponds to the particular input, and an arrangement responsive to the value in the electronic storage means for generating the particular input and using it to drive the illumination device in a way which provides light of the given intensity.
A method of operating a display system as described above includes determining a particular value for controlling the input to an input-driven illumination device and electronically storing the particular value. The method further includes driving the illumination device in response to the particular value in a way which produces light of a desired intensity level, and directing the light of the desired intensity level onto a light modulator.
In a preferred embodiment, the display system includes a light modulator divided into an array of individually controllable pixels and an input-driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto the modulator, depending on the input. The display system further includes a calibrating arrangement for establishing the input for a desired intensity level of the light. The arrangement includes a light sensing mechanism, which senses the light from the illumination device while the illumination device is driven by an initial input. The calibration arrangement is configured to determine a comparison between the sensed light and a value representative of the desired intensity level. The calibration arrangement further includes a control arrangement responsive to the comparison for varying the input so as to provide light of the desired intensity level. The light sensing mechanism may form part of the light modulator.
The input-driven illumination device in either of the aforementioned display systems or the aforementioned illumination assembly may contain one, and only one, light source. Alternatively, the illumination device may include a plurality of light sources, wherein the calibration arrangement is designed to establish the input for a desired intensity level for each light source, so as to produce combined light of a desired color. The particular intensity of light produced by each light source may be different. The desired color may be white. The illumination device may consist of red, green and blue light-emitting diodes.
In the aforementioned display system, the sensing mechanism may be a photodetector. The sensing mechanism may be configured to sense only light within the visible spectrum. The sensing mechanism may be configured to have photopic spectral response substantially similar to the human eye.
A method of operating the immediately aforementioned display system includes providing an input-driven illumination device which is adapted to receive a variable input and which is configured to direct light of variable intensity onto a light modulator depending on the input. The method further includes sensing the light from the illumination device while the illumination device is driven by an initial input and comparing the sensed light to a value representative of the desired intensity. The method further includes establishing the input for a desired intensity level of the light in response to the comparison and directing the light of the desired intensity level onto the light modulator.
In another embodiment similar to the immediately preceding embodiment of a display system, the spectral response of the photodetector may vary from photodetector to photodetector, and the value representative of the desired intensity level is pre-calibrated to vary proportionally with the photodetector spectral response variation. Also, the sensing mechanism may include a plurality of photodetectors, each configured to sense light of a specific range of wavelengths and wherein each range of wavelengths is different.
In another embodiment, a color display includes a light modulator and a plurality of different colored lights, each of which are pre-calibrated to provide light of a given intensity in response to an input of a particular value. The lights are configured to direct the light onto the modulator. This embodiment includes an improvement that includes an electronic storage arrangement for storing the particular value and a control arrangement responsive to the particular value in the electronic storage arrangement for driving the light sources in a way which provides light of the given intensity.